An increasing variety of lighting applications require a precisely controlled spectral characteristic of light. There are many variations of light that appear white. Sunlight, for example, appears warmer than white light from a fluorescent fixture. Light from an incandescent bulb often appears somewhat reddish in color. Yet, humans perceive such lights as ‘white.’ Even for light that appears ‘white’ to the human eye, many applications call for different characteristics of the white light. Typical white light sources provide light of a fixed nature, so that it is often necessary to use a different lighting device for each different application.
It has long been known that combining the light of one color with the light of another color creates a third color. For example, the commonly used primary colors Red, Green and Blue of different amounts can be combined to produce almost any color in the visible spectrum. Adjustment of the amount of each primary color enables adjustment of the spectral properties of the combined light stream. Recent developments for selectable color systems have utilized light emitting diodes as the sources of the different light colors.
Light emitting diodes (LEDs) were originally developed to provide visible indicators and information displays. For such luminance applications, the LEDs emitted relatively low power. However, in recent years, improved LEDs have become available that produce relatively high intensities of output light. These higher power LEDs, for example, have been used in arrays for traffic lights. Today, LEDs are available in almost any color in the color spectrum.
Systems are known which combine controlled amounts of projected light from at least two LEDs of different primary colors. Attention is directed, for example, to U.S. Pat. Nos. 6,459,919, 6,166,496 and 6,150,774. Typically, such systems have relied on using pulse-width modulation or other modulation of the LED driver signals to adjust the intensity of each LED color output. The modulation requires complex circuitry to implement. Also, such prior systems have relied on direct radiation or illumination from the individual source LEDs. In some applications, the LEDs may represent undesirably bright sources if viewed directly. Also, the direct illumination from LEDs providing multiple colors of light has not provided optimum combination throughout the field of illumination. In some systems, the observer can see the separate red, green and blue lights from the LEDs at short distances from the fixture, even if the LEDs are covered by a translucent diffuser. Integration of colors by the eye becomes effective only at longer distances.
Another problem arises from long-term use of LED type light sources. As the LEDs age, the output intensity for a given input level of the LED drive current decreases. As a result, it may be necessary to increase power to an LED to maintain a desired output level. This increases power consumption. In some cases, the circuitry may not be able to provide enough light to maintain the desired light output level. As performance of the LEDs of different colors declines differently with age (e.g. due to differences in usage), it may be difficult to maintain desired relative output levels and therefore difficult to maintain the desired spectral characteristics of the combined output. The output levels of LEDs also vary with actual temperature (thermal) that may be caused by difference in ambient conditions or different operational heating and/or cooling of different LEDs. Temperature induced changes in performance cause changes in the spectrum of light output.
Another problem with existing multi-color LED systems arises from control of the overall system output intensity. In existing systems, to adjust the combined output intensity, e.g. to reduce or increase overall brightness, the user must adjust the LED power levels. However, LED spectral characteristics change with changes in power level. If the light colors produced by the LEDs change, due to a power level adjustment, it becomes necessary to adjust the modulations to compensate in order to achieve the same spectral characteristic.
U.S. Pat. No. 6,007,225 to Ramer et al. (Assigned to Advanced Optical Technologies, L.L.C.) discloses a directed lighting system utilizing a conical light deflector. At least a portion of the interior surface of the conical deflector has a specular reflectivity. In several disclosed embodiments, the source is coupled to an optical integrating cavity; and an outlet aperture is coupled to the narrow end of the conical light deflector. This patented lighting system provides relatively uniform light intensity and efficient distribution of light over a field of illumination defined by the angle and distal edge of the deflector. However, this patent does not discuss particular color combinations or effects.
Hence, a need still exists for a technique to efficiently provide white light of a selectable characteristic. A related need still exists for such a system that does not require complex electronics (e.g. modulation circuitry) to control the intensity of the energy output from the sources of the radiant energy of different wavelengths. A need also exists for a technique to effectively maintain a desired energy output level and the desired spectral characteristic of the combined output as LED performance decreases with age, preferably without requiring excessive power levels.